The purpose of echo cancelling is to compensate a signal for echoes caused by various sources including feedback from a speaker in close proximity to a microphone. In general, prior art echo cancellers use a reference signal to determine the echoes and accordingly compensate the signal by removing (subtracting) an estimate of the echoes from the signal.
However, echo cancelling, either acoustic or line, can be relatively expensive, especially in long delay networks, such as packet-based networks. In traditional echo canceller architecture, the delays in the network are compensated by increasing buffer size and thus memory requirements. Unreliable transport media, such as Internet Protocol networks, have an additional problem of packet loss, which can considerably reduce the effectiveness of an echo canceller.
Referring to FIG. 1, there is shown a block diagram of a conventional echo canceller 100. The conventional echo canceller 100 comprises an echo estimator and control 110 and a subtractor 120. An input signal (Sin 130) is a combination of an Echo 132 (the echoes) and the near end signal. As is known in the art, the echo estimator and control 110 uses the reference signal (Rout or Rin) 134 and the subtractor 120 to remove an estimate of the echo from the input signal 130. The goal of the echo canceller is to create an output signal (Sout 136) that matches the near end signal as closely as possible with the echo sufficiently reduced.
Referring to FIG. 2, there is shown a block diagram of a conventional full duplex hands free (FDHF) echo canceller 200 for a traditional speakerphone. The FDHF echo canceller 200 includes a line echo estimator and control 210 as well as a first subtractor 215 for cancelling line echo 217 (the echoes) introduced by a network (not shown). An acoustic echo estimator is provided along with control 220 and a second subtractor 222 for cancelling acoustic echo 224 between loudspeaker 226 and microphone 228.
Referring to FIG. 3, there is shown a block diagram of a conventional packet network based acoustic echo canceller 300 for connection with a packet network 350. In packet networks, line echo is typically cancelled at IP/PSTN gateways (not shown). The canceller 300 comprises an acoustic echo estimator 300, a subtractor 310, a packetizer 320 and de-packetizer 330.
In the traditional speakerphone, these echo-cancelling resources are located on the phone, which increases the cost for each of the phone sets. These echo-cancelling resources are usually idle, since for most of the time, users are not using the speakerphone feature.
It is therefore desirable to provide an echo cancelling system, which addresses the shortcomings of providing echo cancelling, noted above.